In an electrical communication system, it is sometimes advantageous to transmit information signals (video, audio, data) over a pair of wires (hereinafter “wire-pair” or “differential pair”) rather than a single wire, wherein the transmitted signal comprises the voltage difference between the wires without regard to the absolute voltages present. Each wire in a wire-pair is susceptible to picking up electrical noise from sources such as lightning, automobile spark plugs, and radio stations, to name but a few. Because this type of noise is common to both wires within a pair, the differential signal is typically not disturbed. This is a fundamental reason for having closely spaced differential pairs.
Of greater concern, however, is the electrical noise that is picked up from nearby wires or pairs of wires that may extend in the same general direction for some distances and not cancel differentially on the victim pair. This is referred to as crosstalk. Particularly, in a communication system involving networked computers, channels are formed by cascading plugs, jacks and cable segments. In such channels, a modular plug often mates with a modular jack, and the proximities and routings of the electrical wires (conductors) and contacting structures within the jack and/or plug also can produce capacitive as well as inductive couplings that generate near-end crosstalk (NEXT) (i.e., the crosstalk measured at an input location corresponding to a source at the same location) as well as far-end crosstalk (FEXT) (i.e., the crosstalk measured at the output location corresponding to a source at the input location). Such crosstalks occur from closely-positioned wires over a short distance. In all of the above situations, undesirable signals are present on the electrical conductors that can interfere with the information signal. When the same noise signal is added to each wire in the wire-pair, the voltage difference between the wires will remain about the same and differential crosstalk is not induced, while at the same time the average voltage on the two wires with respect to ground reference is elevated and common mode crosstalk is induced. On the other hand, when an opposite but equal noise signal is added to each wire in the wire pair, the voltage difference between the wires will be elevated and differential crosstalk is induced, while the average voltage on the two wires with respect to ground reference is not elevated and common mode crosstalk is not induced.
U.S. Pat. No. 5,997,358 to Adriaenssens et al. (hereinafter “the '358 patent”) describes a two-stage scheme for compensating differential to differential NEXT for a plug-jack combination (the entire contents of the '358 patent are hereby incorporated herein by reference, as are U.S. Pat. Nos. 5,915,989; 6,042,427; 6,050,843; and 6,270,381). Connectors described in the '358 patent can reduce the internal NEXT (original crosstalk) between the electrical wire pairs of a modular plug by adding a fabricated or artificial crosstalk, usually in the jack, at one or more stages, thereby canceling or reducing the overall crosstalk for the plug-jack combination. The fabricated crosstalk is referred to herein as a compensation crosstalk. This idea can often be implemented by twice crossing the path of one of the differential pairs within the connector relative to the path of another differential pair within the connector, thereby providing two stages of NEXT compensation. Another common technique is to cross the conductors of pairs 1, 2 and 4 (as defined in designation T568B of TIA-568-B.2) and leaving the conductors of pair 3 uncrossed (see, e.g., U.S. Pat. No. 5,186,647), then including a second compensation stage (e.g., in the form of a capacitor) on an attached printed wiring board. This scheme can be more efficient at reducing the NEXT than a scheme in which the compensation is added at a single stage, especially when the second and subsequent stages of compensation include a time delay that is selected to account for differences in phase between the offending and compensating crosstalk. This type of arrangement can include capacitive and/or inductive elements that introduce multi-stage crosstalk compensation, and is typically employed in jack lead frames and PWB structures within jacks. These configurations can allow connectors to meet “Category 6” performance standards set forth in ANSI/EIA/TIA 568B.2-1, which are primary component standards for mated plugs and jacks for transmission frequencies up to 250 MHz.
Alien NEXT is the differential crosstalk that occurs between communication channels. Obviously, physical separation between jacks will help and/or typical crosstalk approaches may be employed. However, a problem case may be “pair 3” of one channel crosstalking to “pair 3” of another channel, even if the pair 3 plug and jack wires in each channel are remote from each other and the only coupling occurs between the routed cabling. To reduce this form of alien NEXT, shielded systems containing shielded twisted pairs or foiled twisted pair configurations may be used. However, the inclusion of shields can increase cost of the system. Another approach to reduce or minimize alien NEXT utilizes spatial separation of cables within a channel and/or spatial separation between the jacks in a channel. However, this is typically impractical because bundling of cables and patch cords is common practice due to “real estate” constraints and ease of wire management.
In spite of recent strides made in improving mated connector (i.e., plug-jack) performance, and in particular reducing crosstalk at elevated frequencies (e.g., 500 MHz—see U.S. patent application Ser. No. 10/845,104, entitled NEXT HIGH FREQUENCY IMPROVEMENT BY USING FREQUENCY DEPENDENT EFFECTIVE CAPACITANCE, filed May 4, 2004, the disclosure of which is hereby incorporated herein by reference), channels utilizing connectors that rely on either these teachings or those of the '358 patent can still exhibit unacceptably high alien NEXT at very high frequencies (e.g., 500 MHz). As such, it would be desirable to provide connectors and channels used thereby with reduced alien NEXT at very high frequencies.
One solution is offered in co-pending and co-assigned U.S. patent application Ser. No. 11/044,088 (the '088 application), filed Mar. 25, 2005, the disclosure of which is hereby incorporated herein. The '088 application proposes to reduce the conversion of differential to common mode crosstalk by eliminating crossovers between pairs 1, 2 and 4 of the conductors of a connector and introducing a crossover only in pair 3. This solution can reduce the conversion of differential to common mode crosstalk considerably. However, such an arrangement may fail to compensate for common mode crosstalk induced on pair 3 when either of pairs 2 or 4 is differentially excited, and further may fail to compensate for differential to common mode crosstalk between pairs 1 and 2 and pairs 1 and 4. Moreover, the open loop configurations of pairs 1, 2 and 4 can generate and render the connector susceptible to electromagnetic interference.